The present invention relates to a method and apparatus for the detection and location of subterranean anomalies, and particularly, to such method and apparatus for detecting tunnels or other void spaces within the earth.
Geological surveying by electromagnetic means has been done for many years. The techniques that have been used involved the detection of changes in the earth's resistivity by measurements in mutual coupling between a transmitter antenna (i.e., a long wire or small horizontal loop) and a receive antenna located on or near the earth's surface. This technique works good for large anomalies (i.e. gravel pits, large oil or ore deposits, etc.) and small anomalies located near the earth's surface. These techniques involve an averaging of the earth's resistivity and how much an anomaly will change this total average. Other techniques involved are the use of DC probes and the plotting of the potential lines. This technique is slow and is limited to shallow anomalies. Another method involved the drilling of two boreholes parallel to one another and doing transmission line experiments. This technique is useful but expensive to do considering the cost of drilling.